The present invention particularly relates to electrocardiogram (ECG) transducers for a fetal monitor, wherein the ECG may either be a direct ECG of the fetus or a maternal ECG. A fetal monitor measures and records the fetal beat-to-beat heart rate (FHR) and uterus activity (toco). Simultaneous assessment of both parameters facilitates an accurate determination of the fetal condition. The FHR may be obtained with a scalp electrode after rupture of the membranes; prior to this time an ultrasound (US) transducer placed externally on the pregnant woman's abdomen may be used. Uterus activity may likewise either be recorded with an intrauterine pressure transducer or by an external tocodynamometer (a tension-measuring device). Some external methods for recording the FHR depend on the placement of the transducers. E.g., the accuracy of the ultrasound method is dependent on the proper orientation of the ultrasound beam; if the beam is not properly focussed on the fetal heart, the maternal heart rate may be recorded instead of the fetal heart rate. It is therefore advantageous to record the maternal heart rate and to compare it with the fetal heart rate.
The present invention deals particularly with the fetal and/or maternal ECG electrodes of such fetal monitors, and in fact the invention was made during the design process of a fetal monitor. It is understood, however, that the invention is not limited to such an application, but rather relates to all types of transducers with galvanic coupling to the patient, including, e.g., electrocardiogram transducers, electroencephalogram transducers, electromyogram transducers and electrooculogram transducers.
A common characteristic of the transducers mentioned above (i.e., transducers with galvanic coupling to the patient) is that they provide an electrical connection between the patient and the monitor. For reasons of patient safety, the patient ground potential (floating ground) must therefore be galvanically separated from the ground potential of the associated monitor (non-floating ground). This problem has been solved in the past with separation circuitry on a printed circuit board inside the monitor, e.g., with transformers or optoelectronic couplers.
The use of separation circuitry placed inside the monitor for separating the patient's floating ground potential from the monitor's non-floating ground potential has several drawbacks. One drawback is that, since there is capacitive coupling between the mains and the patient and between non-floating ground and the patient, an AC voltage of the mains frequency (50 Hz/60 Hz) is coupled into the transducer cable. This common mode voltage impairs the results of the measurement. Moreover, since the fetal QRS complexes must be recorded very accurately and their signal level is typically quite low (e.g., 20 .mu.V peak to peak in some cases), fetal monitoring requires a high common mode rejection ratio (CMRR). A reliable signal can be obtained only if the common mode rejection ratio is greater than 150 dB. It is technologically possible to improve the CMRR by approximately 30 dB by means of a suitable notch filter; the remaining 120 dB must be attained by other measures.
One method for attaining the necessary CMRR is to apply a predefined potential to the patient, i.e., to drive the patient to this potential. The attainable CMRR is around 103 dB; the remaining 17 dB necessary for a CMRR of 120 dB can be obtained by means of an active variable-gain amplifier (common-mode amplifier) which drives the electrode. For stability reasons, this common-mode amplifier can usually not compensate for considerably more than 20 dB. This technique is widely used in medical monitoring of adults, e.g., for recording the maternal heart rate in fetal monitoring applications. As the active electrode driving the patient is usually an electrode attached to the right leg of the patient, it is also known as "right leg drive." To acquire a direct ECG of the fetus, a silver plate is attached to the mother's body. This method has some serious disadvantages, including:
1. The method works properly only if the right leg electrode contains silver, which makes the electrode expensive, and if electrode gel is used between the electrode and the patient's skin (to reduce the resistance between electrode and skin). The gel is uncomfortable, expensive and requires periodic, difficult cleaning of the equipment.
2. An additional electrode, which is required only to compensate for an imperfect measurement technique, must be applied. This is particularly disturbing if the electrodes do not remain attached to the patient for long periods (such as in intensive care monitoring), but rather are changed frequently (as in fetal screening). It is also disturbing if the other electrodes are not attached to the patient's body (as with a fetal scalp electrode), in which case the right leg electrode must be handled separately from the measuring electrodes.
An important goal of electrode design is therefore to make the electrode gel unnecessary, and, in some cases, to altogether avoid the right leg electrode. This goal has been extremely difficult to achieve. E.g., when the reference electrode (right leg drive) is omitted, the CMRR is approximately 61 dB, instead of the required 120 dB; on the other hand, when the reference electrode is used and the electrode gel omitted, a common-mode amplifier with an amplification of at least 40 dB at 50/60 Hz is required, which requires difficult measures to keep the amplifier stable under all operating conditions.
Due to the safety risks associated with common-mode signals, transducers which generate a floating potential (such as ECG transducers) and transducers which generate a non-floating potential (such as toco transducers) cannot not be connected with the associated monitor via a single cable. This is a significant limitation, as there are many applications employing multiple transducers where it would be desirable to have only one cable leading to the monitor, e.g., direct fetal ECG and toco monitoring, or maternal ECG and ultrasound monitoring.
The schematic diagram in FIG. 1a illustrates the problem underlying the present invention. Patient 1 is being monitored; an electrode 2 with an electrical connection to the patient is used (electrode 2 represents e.g., an electrocardiogram electrode). There is a certain capacity C.sub.1 between the patient's body and the mains voltage U.sub.s. Likewise, there exists a capacity C.sub.2 between the patient and ground. This situation generates a common-voltage mode voltage U.sub.p0 at electrodes 2. (It is a "common-mode" because it is the same for other electrodes.) Common-mode voltage U.sub.p0 impairs the measurement.
The situation is also shown, in the form of an equivalent circuit, in FIG. 1b. The circuit of FIG. 1b can be transformed into the circuit of FIG. 1c, i.e., common-mode voltage U.sub.p0 can be calculated as ##EQU1## C.sub.1 is typically in the range of 20 pF, whereas C.sub.2 is typically approximately 200 pF. For mains voltage U.sub.s between 100 V and 240 V, common-mode voltages U.sub.p0 between 9.1 V and 21.8 V are obtained.
FIG. 2 is a schematic diagram illustrating an artificial measurement using two electrodes 5, 6. Common-mode voltage U.sub.p0 (indicated by voltage source 3) is applied to capacitor 4 (C.sub.p) that represents the internal resistance of the voltage source (i.e., C.sub.p is equal to the parallel combination of C.sub.1 and C.sub.2 of FIGS. 1a-1c), thus generating a voltage U.sub.p. Voltage U.sub.p is a common-mode voltage present at both electrodes; as shown below, U.sub.p is not equal to U.sub.p0. Electrode 6 contains an imbalancing impedance Z.sub.im consisting of a 51.1 k.OMEGA. resistor 7 and a 47 nF capacitor 8. Z.sub.im is prescribed by international standards, e.g., IEC 62D, to simulate the different electrode impedances in an artificial environment.
The two electrode signals are fed to a difference amplifier 9. The input resistance of this amplifier is represented by resistors 10 and 11. The input resistance R.sub.i at both electrode inputs is approximately equal, about 10M.OMEGA..
The circuit 12 surrounded by dashed lines operates at floating potential, indicated by arrow 13, as opposed to earth potential 14. The floating circuit therefore has a capacitive coupling to ground (i.e., earth potential), which coupling is represented by capacitor 15 of capacitance C. Capacitance C results from the cable and circuit capacitance to ground and is approximately 200 pF. Capacitor 4 (C.sub.p) and capacitor 15 (C) constitute a capacitive voltage divider, which is why voltages U.sub.p0 and U.sub.p are not equal.
The CMRR of the circuit of FIG. 2 is calculated according to the following equation: ##EQU2## wherein i.sub.CM is the common-mode current flowing through the two electrodes. i.sub.CM is approximately equal to two times the current through each of the electrodes, i.e., EQU i.sub.cm .apprxeq.2*i.sub.i .apprxeq.2*i.sub.2 ( 3)
Using eq. (2), the circuit of FIG. 2 reveals ##EQU3## which yields, assuming a mains frequency of 60 Hz, a capacitance C of 200 pF and internal resistance R.sub.i of 10 M.OMEGA., a common-mode rejection ratio of EQU CMRR.apprxeq.61 dB (5)
Since 59 dB of the 120 dB required to obtain a reliable signal is missing, the circuit of FIG. 2 is not suitable for monitoring a patient.
FIG. 3 is a schematic circuit similar to FIG. 2, but including a reference electrode. In addition to the components shown in FIG. 2 (which are labeled with the same reference numbers in FIG. 3), the circuit of FIG. 3 comprises a common-mode amplifier 16 connected via an impedance Z.sub.LP to an additional reference electrode. The reference electrode is typically applied to the patient's leg.
Impedance Z.sub.LF may be characterized by the parallel combination of a resistor 17 and a capacitor 18 and represents the impedance between the patient's body and the reference electrode. If the reference electrode comprises silver and electrode gel is used, then Z.sub.LP is given by EQU Z.sub.LP .apprxeq.Z.sub.im ( 6)
If the reference electrode is directly coupled to floating ground, the ratio R.sub.i : 2 : Z.sub.LF determines the relation (i.sub.i +i.sub.2)/i.sub.3 (i.sub.3 represents the current through the reference electrode); the common-mode current fraction (i.sub.i +i.sub.2) will therefore decrease (assuming a mains frequency of 60 Hz) by the factor ##EQU4## which corresponds to a further common-mode rejection of 42 dB. If the active amplifier (common-mode amplifier 16) contributes another 20 dB, the total common-mode rejection ratio is EQU CMRR.sub.total =61 dB+42 dB+20 dB=123 dB (8)
which is, according to the requirement of 120 dB, a sufficient value. This, however, requires the use of electrode gel at the reference electrode, which has the disadvantages mentioned above. If no gel is used, the common-mode amplifier must provide an amplification of 40 dB, which, due to stability requirements, is difficult to achieve.
A further problem arises if the monitor provides a single input port for alternative insertion of a transducer which provides non-floating potentials and a transducer which provides floating potentials, e.g., the ultrasound transducer and the scalp electrode, each of which is used to detect fetal heart rates. As these transducers are not used together at the same time, it is possible to insert them alternatively into the same port, or connector, thus saving a second connector. To ensure proper operation, the connector must be divided into a floating part and a non-floating part. This in turn means that the shield of the ECG cable cannot be fed through the connector, resulting in capacitive leakage around the connector. Common-mode currents that cannot be compensated by a common-mode amplifier can flow through this leakage. The attainable CMRR is around 90 dB in this case. Although it is possible to attain another 30 dB by additional measures, these require either components with extremely small tolerances or additional time-consuming adjustments during the manufacturing process and are therefore very costly. This problem is schematically illustrated in FIG. 4. If one uses a connector for insertion of a floating (e.g., ECG) as well as a non-floating (e.g., ultrasound) transducer, the shield of the ECG cable cannot be fed through the connector, thus generating a capacitive leakage which cannot be offset by an active amplifier. This capacitive leakage is indicated in FIG. 4 by capacitors 19, 20 and 21, each of which has a value C.sub.c of approximately 2 pF.
It is therefore a goal of the present invention to provide a transducer which makes it unnecessary to use electrode gel and, at least in some instances, to avoid the reference electrode altogether. A further goal of the present invention is to provide a means for safely combining transducers providing floating potentials with transducers providing non-floating potentials. A still further goal of the present invention is to provide a means for avoiding capacitive leakage around the input port of the monitor to which the transducer(s) is (are) connected.